1. Field of the Invention
The present invention pertains to a method for installing and/or removing cables in cable conduits. It relates more particularly not only to the installation and/or removal of optic fiber cables but also to the removal of copper cables which are in most conduits and may be retrieved for other uses. The invention furthermore relates to a device for the implementation of this method.
The standard methods of installing cables in cable conduits are often very costly and lengthy to implement, and can lead to rapid deterioration of the cable due to its friction against the internal walls of the conduit. A standard method shown in the diagram of FIG. 1 consists in placing a winch T at one end of the course and placing a reel B of cable 2 at the other end of the course. Then the line 1 of the winch T is made to pass into the conduits C, and then the end of the line 1 is hooked to an end of the cable 2 to be inserted. The cable 2 is then installed in the conduits C by pulling, in actuating the winch T in the direction of the arrow shown in FIG. 1. Furthermore, tracked systems 3 must necessarily be placed at regular intervals along the course in order to hold and guide the cable 2. These tracked systems 3 are therefore placed in underground chambers Ch1, Ch2, Ch3, Ch4 which are generally spaced out from one another at distances of 300 m. The continuous motion of these tracked systems 3 must furthermore be synchronized with the pulling speed dictated by the winch T. Consequently, this method calls for the presence of a workman at each active station, namely at each end of the path and in the underground chambers in which intermediate equipment is provided for the efficient running of the operations.
This method therefore calls for the use of heavy, bulky, and often costly equipment and the mobilizing of large numbers of workmen. Furthermore, to prevent the cable that is to be installed from deteriorating through friction against the walls of the conduits, the course taken has to be relatively rectilinear. The length of the course is also limited by the length of the winch line, the number of workmen to be deployed and the shape of the path to be made between the telephone central and the subscriber. This path necessarily includes curves and is subdivided into rectilinear sections. The maximum length of these sections is generally about 1500 m. In a 1500 m section of this kind, the method requires the presence of six workmen on the site.
Furthermore, this method requires the injection of lubricants to prevent excessively rapid deterioration of the cable through friction against the walls of the conduits. However, in this method, the deterioration of the cable is still inevitable because friction cannot be completely avoided.
Thus, the conventional way of installing a cable in a conduit takes a long time and is very costly, especially because there is a heavy deployment of workmen. Furthermore, the course defined by the conduits has to be subdivided into small relatively rectilinear sections. The cables must furthermore have a very low conduit fill rate, typically less than 20%, in order to prevent, as far as possible, friction against the walls of the conduits.
The fill rate is defined as the ratio between the diameter of the cable and the internal diameter of the conduit.
Other methods have been devised in recent years to improve the conventional methods, especially to reduce their cost and shorten the implementation time. These methods are based on the xe2x80x9cfloatingxe2x80x9d technique. In this method, a current of water is conveyed into the conduits in order to facilitate the transportation of the cable and reduce risks of friction of the cable against the wall. With a flow of water being created, the cable naturally tends to get centered in the flow of water.
2. Description of the Related Art
The patent application EP-A-0 264 767 describes a method for the introduction of a submerged optical cable into a conduit by means of friction of water on the cable. In this case, the cable has a conduit fill rate that is relatively low, since it is about 14%, and prevents friction against the walls of the conduit to the maximum extent. This method furthermore requires the use of floats that are placed regularly along the cable. These floats not only provide a better support for the cable but also improve the movement of the cable owing to the friction of water on them. However, nothing in this document says whether a regulation of the water flow rate is needed and there is even less of an indication of how to achieve a regulation of this kind. The progression of the cable is obtained solely through the friction of water on the cable and the floats. This method is therefore limited since it cannot be used to install cables with a very unfavorable conduit fill rate which may go up to 90% nor can it be used for installation on a highly winding course with a length of over 2000 m. Furthermore, this method cannot be used to remove cables occupying the conduits.
The patent application WO 94/09538 describes a method for the introduction of a cable into conduits within which there flows a current of water. According to this method, the cable is moved by mechanical assistance. In fact, this method is based on the technique of floating combined with pulling. Indeed, it consists in placing a plunger on the front end of the cable. The plunger is actuated by the pressure of water flowing in the conduit so as to pull the cable. A system designed simply to guide the cable so that it is accurately introduced into the conduit and centered in the flow of water is planned at the input of the conduit.
In this case, a variation of the pressure of water leads to a variation in the pulling force of the plunger and therefore a variation of the speed of movement of the cable. However, the pulling operation does not provide for efficient holding and efficient guiding of the cable over a great length. Furthermore, an excessive variation in the water pressure leads to a sudden variation in the speed of movement of the plunger and therefore affects the steadiness of the cable in the water current. There is a risk indeed that the cable may get swayed out of line to a greater or lesser extent and rub against the walls of the conduits, especially at the connections between the conduits or in the curvatures, thus causing a deterioration of its sheath.
The flow rate and water pressure are not constant all along the course. They are greater at the point where the water is injected and the cable is introduced, and they get smaller as and when the cable moves forward in the conduit and moves away from the injection point. It is therefore necessary to increase the water pressure as and when the plunger moves away, in order to keep a satisfactory speed of progression of the cable. However, this pressure should not be excessive in order to avoid pushing the cable against the walls of the conduit.
This method therefore cannot be used to install cables with a high fill rate of up to 90%. Furthermore, it is not suited to highly winding courses over lengths of at least 2000 m.
The patent application WO 92/17927 describes another method for the installation of cables in conduits by pressure from a fluid (water) flowing in these conduits.
The method described in this document comprises mechanical means (a reel) for paying out the cable at a speed that is as uniform as possible. When the first end of the cable is inserted into the first end of the conduit, a fluid (water) is simultaneously injected into the conduit in order to exert a flotation thrust and a drive force on the cable, enabling the cable to reach the other end of the conduit. The flow rate of the fluid therefore depends on the type of cable that has to travel through the conduit, and its value is computed as a function of the power needed to drive the cable.
This document does not describe any means to keep the flow rate of water constant in the conduit. It is on the contrary the speed of progression of the cable that is kept as constant as possible.
The method described in this document can be used to install a cable in lengthy, winding conduits but does not give a very high fill rate. Indeed, since the flow rate is not kept constant, the cable may be subjected to high pressure or sway out of line and rub against the walls of the conduit when there is a change in the flow rate. A fill rate of 90% would therefore be far too risky for the cable that is to be installed.
Furthermore, the methods that have just been described cannot be used to remove copper and/or optic cables occupying conduits without harming them. Now, it may be worthwhile to remove these cables and keep them intact for use in other applications.
The present invention overcomes the above-mentioned drawbacks. Indeed, it proposes a method for the introduction and/or removal of a cable in conduits defining a highly winding course with a length of over 2000 m, the cable having a very unfavorable fill rate configuration that can go up to 90%, wherein the cable is moved no longer by a pulling effect but by a thrust effect to which it is subjected.
The invention relates more particularly to a method for the introduction and/or removal of a cable in conduits for the passage of cables within which a current of water is conveyed, the cable having a conduit fill rate that can reach 90% and the conduits forming a winding course on a length of over 2000 m, characterized in that the cable is subjected to a thrust force that controls its speed of progression and in that the thrust force and the pressure of water flowing in the conduits are regulated so as to maintain a constant water flow rate within the conduits.
Since the water flow rate is constant and controlled in the conduits, the cable is borne in the same way along the conduits, thus considerably limiting the risks of impacts against the walls of the conduits.
The method according to the invention is fast and removes the need to use intermediate assistance. It requires the presence of only one workman to install a cable in conduits defining a winding course with a length of over 2000 m, capable of reaching 5 kilometers or even more depending on the rate of occupation of the conduits. Consequently, the method of the invention costs little. It can furthermore be used to install the cables according to the demand from subscribers. The method of the invention is therefore economically profitable.
Another object of the invention pertains to a device for the implementation of the method according to the invention. This device has the advantage of being compact and positioned on a single trailer towable by a light vehicle. It is therefore easy to move on a site. It is more particularly characterized in that it comprises:
thrust means to provide for the progression of the cable,
a heat engine capable, through a selection switch system, of controlling either a hydraulic pressure circuit, said circuit being connected to a water tank and to an injection gun to inject water into the conduits, or an air compression circuit, said circuit being connected to said injection gun to inject compressed air into the conduits after the laying of a cable in order to drain them,
sensors to measure the lengths and speed of progression of the cable and a strain gauge to measure the force exerted on the cable, said sensors and said gauge being linked to:
a central processing unit capable of managing the commands that are sent to it to drive the thrust means and the heat engine,
and in that said device is positioned on a trailer towable by a light vehicle.